The gas-phase polymerization of conjugated diene monomers has been accomplished by employing solid-supported anionic catalyst. For example, U.S. Pat. No. 6,359,087 teaches gas-phase anionic polymerization techniques that employ catalysts having the formula P(Me)n, where P is a metalatable particle having a diameter of about 1 micron to about 1000 microns comprising a bound rubber. The particle is multiply-metalated with “n” covalently bonding Group IA alkali metal (Me) atoms. The bound rubber particle may comprise any metalatable carbon black-bound rubber, such as carbon black-styrene butadiene rubber, carbon black-butadiene rubber, carbon black-natural rubber, and the like. These anionic polymerization initiators, when charged into the reaction zone of a gas-phase apparatus, are capable of anionically homopolymerizing conjugated diolefin monomer having about 4 to about 12 carbon atoms and copolymerizing conjugated diolefin monomers and monovinyl aromatic monomers having from about 8 to about 20 carbon atoms to form very high molecular weight branched diene polymers. The resulting polymers are macro-branched and are characterized by being light and granular.
While the prior art contemplates copolymerization of vinyl aromatic monomer with conjugated diene monomer in the gas phase, formation of a vinyl aromatic gaseous stream is difficult. As those skilled in the art appreciate, vinyl aromatic monomer, such as styrene, will self-polymerize upon heating. Under atmospheric pressure, this heat triggered self-polymerization occurs at temperatures below the boiling temperature of styrene. Therefore, a reduction in the vaporization temperature is required, but even under vacuum, the formation of the gas stream of styrene can be problematic to achieve.
As noted above, the polymer product produced by known gas-phase anionic polymerization techniques are macro-branched diene polymers that are light and granular. While this may be advantageous in many instances, these macro-branched diene polymers may be disadvantageous in other instances. That is, it may be desirable to produce, by gas-phase polymerization, conjugated diene polymers that are linear and uncoupled.